The present invention relates to esters containing more than one ester linkage, hereinafter known as xe2x80x9ccomplexxe2x80x9d esters, to formulations comprising one or more of these complex esters and to various uses of the complex esters and the formulations. More specifically, the present invention relates to complex esters and their use as an additive and/or a base fluid and/or thickener in various types of formulations suitable for use in lubrication applications, for example gear oils, hydraulic fluids, compressor oils, greases and four-stroke oils. The present invention also relates to formulations comprising one or more of these complex esters.
Complex esters are known in the art. For instance, DE-A-2620645 discloses a process for lubricating a two stroke engine by using a two stroke lubricating oil of which the base oil consists of at least one hydrocarbon oil and a complex ester. The complex ester results from esterification of trimethylolpropane with at least one saturated, linear or slightly branched C4-C36 saturated, aliphatic dicarboxylic acid and a mixture of at least one linear or slightly branched C2-C14 monocarboxylic acid and at least one saturated, linear or slightly branched aliphatic C15-C30 monocarboxylic acid. Maximum kinematic viscosity at 98,9 C (Vk,98.9) of the complex ester suitably is only 25 cSt, which corresponds to a typical viscosity of a two-stroke oil.
In FR-A-2,187,894, a process for lubricating two stroke engines or rotary engines is disclosed, wherein use is made of a lubricating oil of which the base oil is a complex ester having a kinematic viscosity of more than 6 cSt at 98,9 C. In this reference complex esters are defined as esters formed by condensation of a polycarboxylic acid with a mono- and polyalcohol or as esters"" formed by condensation of a polyol with a poly- and monocarboxylic acid. Several examples of complex esters are given: adipate/trimethylolpropane/heptanol having a Vk,98.9 of 19,2 cSt, adipate/trimethylolpropane/dodecanoic acid having a Vk,98.9 of 13,7 cSt and azelaic acid/pentaerythritollheptanoic acid/dodecanoic acid having a Vk,98.9 of 15,4 cSt. Again, these low viscosities are typical for two-stroke engine oils.
DE-A-2130850 discloses a lubricant composition containing or consisting of at least one low viscosity and one high viscosity component, where the high viscosity component is a complex ester having a kinematic viscosity at 99 C of more than 50 cSt and a flat viscosity-temperature behaviour. The complex esters are obtained by esterification of unbranched dicarboxylic acids having at least 10 carbon atoms with tri- or tetrafunctional alcohols and stopping with monoalcohols of which at least 25% is linear and low molecular. Trimethylolpropane and pentaerythritol are listed as suitable alcohols, whilst n-butanol and n-hexanol are mentioned as suitable low molecular monoalcohol chain stopping agent.
It has been found that complex esters having improved properties can be obtained by selecting certain compounds for use in the production of the complex ester so as to reduce or remove the number of free alcohol and/or carboxylic acid groups in the ester and so terminate the esterification process. Such compounds are hereinafter referred to as xe2x80x9cchain stopping agentsxe2x80x9d. We have found that monoalcohols having relatively long carbon chains, i.e. of 14 carbon atoms or more, or monocarboxylic acids having at least 7 carbon atoms provide surprising improvements in properties of the complex esters.
In WO-A-97108277 two categories of ester base stocks for smokeless two stroke engine lubricants are disclosed. The first category are ester base stocks comprising a first ester having a viscosity at 100 C of 2 cSt or less and a second ester having a viscosity such that when the fust and second ester are mixed, the resulting mixture has a viscosity at 100 C of 3.0 to 20.0 cSt and a smoke index of at least 75. The second ester may be a stopped, i.e. chain terminated, or unstopped, i.e. still having some functionality, complex ester. The second category of ester base stocks is formed by one or more esters selected from the group consisting of (a) linear oligoesters having a molecular weight of 3000 Daltons or less, (b) complex, non-hindered polyesters wherein the polyol is a molecule having one or more beta hydrogen atoms, (c) complex, non-hindered polyesters wherein the polyol component is a non-hindered polyol having at least 3 OH groups and (d) esters wherein the polyol component is a hindered polyol and the carboxylic acid is a mono- or polycarboxylic acid or a mixture thereof. Several complex esters of the various categories are described, but most of them have a relatively low kinematic viscosity. The stopped complex ester having the highest kinematic viscosity at 100 C (44,5 cSt) is an ester of trimethylolpropane, dimer acid and oleic acid (C18:1 monoacid) as the chain stopping agent.
However, it has been found that the use of dimer acid, i.e. mainly dimerised fatty acids also comprising some trimerised fatty acids, as the sole polycarboxylic acid component has some disadvantages in terms of interaction with certain additive packages comprising sulphur- and/or phosphorus-containing components. Therefore, it would be advantageous to provide a complex ester not comprising dimer acid as the sole polycarboxylic acid component. Furthermore, it would be advantageous if such stopped complex esters could be provided having high kinematic viscosities at 100 C, i.e. 30 cSt or higher.
The present invention aims to provide a complex ester having a relatively high viscosity, which can be used as a functional fluid itself or in various formulations as a functional fluid, for example a lubricating formulation. Furthermore, and depending on the application, the complex ester should provide high oxidation stability and excellent lubricity, whilst, desirably, possessing good biodegradability characteristics. It will be appreciated that the latter is highly desired in view of the increasing environmental awareness and corresponding demand for environmentally friendly products.
Accordingly, the first aspect of the invention relates to a complex ester obtainable by an esterification reaction between at least one polyfunctional alcohol and at least one polyfunctional carboxylic acid and a chain stopping agent, wherein
(a) the polyfunctional alcohol is a hindered or non-hindered, aliphatic polyol,
(b) the polyfunctional carboxylic acid comprises an aliphatic dicarboxylic acid containing from 9 to 18 carbon atoms, dimerised and/or trimerised fatty acids or mixtures thereof, with the proviso that dimerised and trimerised fatty acids do not constitute more than 80% by weight, preferably not more than 50% by weight, of the total amount of polyfunctional carboxylic acid used,
(c) the chain stopping agent comprises either an aliphatic monocarboxylic acid selected from the group consisting of straight chain saturated acids containing from 7 to 22, preferably from 7 to 14, carbon atoms, branched saturated acids containing from 7 to 24 carbon atoms, straight or branched unsaturated acids containing from 16 to 24 carbon atoms and mixtures thereof or at least one aliphatic, straight or branched, saturated or unsaturated, monofuntional alcohol containing at least 14 carbon atoms, and preferably not having more than 24 carbon atoms, and
(d) the complex ester has a kinematic viscosity at 100 C (Vk,100) of from 30 to 1000 cSt, preferably from 30 to 200 cSt.
Preferably the complex ester according to the first aspect of the invention is obtained by an esterification reaction between at least one polyfunctional alcohol and at least one polyfunctional carboxylic acid and a chain stopping agent
The polyfunctional alcohol preferably is a hindered polyol, more preferably a neopentyl polyol. Examples of suitable neopentyl polyols are neopentyl glycol, dipentaerythritol, trimethylolpropane and pentaerythritol, the latter two being particularly preferred.
The polyfunctional carboxylic acid preferably comprises at least one aliphatic dicarboxylic acid having from 9 to 12 carbon atoms, more preferably selected from nonanedioic acid, decanedioic acid, dodecanedioic acid and mixtures thereof. The presence of dimerised and/or trimerised fatty acids is also considered beneficial provided the amount of such acids does not exceed 80% by weight, preferably 50% by weight, of the total amount of polyfunctional carboxylic acids used. Dimerised and/or trimerised fatty acids may be obtained by subjecting an unsaturated fatty acid-containing feedstock to dimerisation by heat treatment in the presence of a suitable catalyst, as is well known in the art. Suitable unsaturated fatty acid containing sources usually comprise a mixture of unsaturated fatty acids with oleic acid (C18:1) often being the main component beside other mono- and polyunsaturated fatty acids. Dimer acid (xe2x80x9cC36dixe2x80x9d) is produced in substantial quantities in the dimerisation reaction. The final product, which is used for manufacturing the complex esters of the invention, usually is a mixture of dimers and trimers commonly in a dimer/trimer ratio of about 80/20. This mixture contains aliphatic as well as cyclic structures including both naphthenic and aromatic structures. If desired, dimers and/or trimers of high purity (e.g. 95% or more) can be manufactured by molecular distillation of the aforementioned mixture of dimers and trimers. This mixture of dimers and trimers as well as purified dimers and/or trimers can be used as the dimerised and/or trimerised fatty acid component. If desired, the dimerised and/or trimerised fatty acid(s) used can be subjected to hydrogenation prior to being used for forming the complex ester.
Suitably, the polyfunctional carboxylic acid is not dimerised and/or trimerised acid alone, as it was found that this may affect the oxidation performance of for instance a gear oil formulation. It was found that a maximum level of 80% by weight of dimerised and/or trimerised acid, based on total weight of polyfunctional carboxylic acid used, still results in an acceptable oxidation stability. The best results are, however, attained when the dimerised and/or trimerised acid does not constitute more than 50% by weight, more preferably not more than 35% by weight, of the total amount of polyfunctional carboxylic acid used.
The chain stopping agent is used to react with the reactive OHxe2x80x94 or COOH-groups, as may be the case, which remain unreacted after reaction between the polyfunctional alcohol and the polyfunctional carboxylic acid. The chain stopping agent should preferably have a relatively long carbon chain for achieving optimum viscosity properties (i.e. a kinematic viscosity at 100 C of at least 30 cSt). In those applications where oxidation stability is very important, such as in gear oil formulations, the chain stopping agent preferably should be saturated. For applications where oxidation stability is less critical, such as for instance in hydraulic fluids, unsaturated fatty acids like olein (technical grade oleic acid) or unsaturated alcohols may also be used. Of the chain stopping agents mentioned above, isostearic acid (isoC18) is very much preferred. However, other fatty acids, like palmitic acid (C16) or stearic acid (C18) are also useful. Furthermore, monocarboxylic acids such as octanoic acid and decanoic acid can also be used. Guerbet acids are also included among the suitable monocarboxylic acids. Examples of suitable monofunctional alcohols are tetradecanol, isotetradecanol, octadecanol and iso-octadecanol. Guerbet alcohols are also included among the suitable monofunctional alcohols.
The complex ester according to the present invention should have a Vk,100 of from 30 to 1000 cSt and preferably from 30 to 200 cSt. For certain applications, such as in gear oils, it is preferred that the Vk,100 has a value of from 100 to 140 cSt. The kinematic viscosity at 40 C (Vk,40 of the complex esters suitably has a value in the range of from 230 to 20,000 cSt, more suitably from 230 to 2800 cSt.
The polyol, polyfunctional carboxylic acid(s) and chain stopping agent, which react to form the complex ester, are preferably used in the following amounts depending in the specific materials employed (xe2x80x9cpbwxe2x80x9d are parts by weight):
15-20 pbw of polyol,
20-25 pbw polyfunctional carboxylic acid and
55-65 pbw chain stopping agent.
The materials are selected so as to provide a complex ester having a Vk,100 within the preferred range of from 100 to 140 cSt.
The complex ester according to the present invention can suitably be used in combination with an extreme pressure and/or anti-wear additive (hereinafter EP/AW) containing sulphur and/or phosphorus-containing compounds e.g. in gear oils.
Accordingly, a further aspect of the invention invention relates to a formulation comprising a complex ester as described according to the first aspect of the invention and a sulphur and/or phosphorus-containing EP/AW additive package in a weight ratio of complex ester to additive package of from 1:3 to 9:1. Suitable sulphur and/or phosphorus-containing EP/AW additive packages are well known in the art, particularly for use in gear oils to avoid wear of the gear wheels. Commercially available sulphur-phosphorus-containing EP/AW additive packages are, for instance, manufactured by Ethyl Corporation, Lubrizol and Paramins.
The complex ester according to the invention can be used as a functional fluid in many different applications, for example in lubricating formulations. The ester may be used as a functional fluid or as an additive and/or a base fluid and/or as a thickener in a functional fluid composition.
Thus, the present invention also relates to the use of the complex ester described according to the first aspect of the invention as a functional fluid.
The present invention also relates to functional fluid compositions comprising the complex ester described according to the first aspect of the invention.
The invention also relates to the use of a formulation containing the complex ester as described in the first aspect of the invention as functional fluid composition, such as transmission oils, for example automotive and industrial gear oils, axle oils and automatic transmission fluids, and also in hydraulic fluids, four-stroke oils, fuel additives, compressor oils, greases, chain oils and for metal working and metal rolling applications.
Examples of functional fluids and functional fluid compositions include transmission oils, for example automotive and industrial gear oils, axle oils and automatic transmission fluids, and also in hydraulic fluids, four-stroke oils, fuel additives, compressor oils, greases, chain oils and for metal working and metal rolling applications.
It has been found that the complex ester according to the invention is particularly suitable to be used as a high viscosity base fluid and/or a thickener in multigrade gear oil formulations.
Multigrade gear oil formulations comprising a synthetic thickener are known in the art. Common synthetic thickeners are polyisobutylene (PIB), VI improvers, such as poly(methyl)methacrylate, olefin copolymers and the like, and polyalphaolefins (PAO) having a high kinematic viscosity. An example of a PAO thickener is PAO 100, i.e. a PAO having a Vk,100 of about 100 cSt. Such high viscosity PAO is used to obtain the multigrade properties and the desired viscosity, whilst maintaining thermal and oxidation stability. In addition to such PAO a low viscosity ester is normally used to improve the solubility and compatibilty of the additives used, to enhance thermal stability and oxidation stability and to impart the desired low temperature viscosity to the gear oil formulation. An EP/AW additive package is applied to avoid wear of the gear wheels. Finally, a low viscosity (i.e. Vk,100 of 4-10 cSt) PAO, also denoted as PAO 4 to PAO 10, and/or a mineral oil having a high viscosity index (VI) is normally present as a base fluid. In case a fully synthetic multigrade gear oil is desired, a low viscosity PAO is used.
It has been found, however, that although the current synthetic multigrade gear oils containing a synthetic thickener perform satisfactorily in a number of demanding applications, there is still a need for improvement to cope with the increasing requirements of modern gear oils such as for heavy duty commercial vehicles and for passenger cars with long drain intervals or filled for life systems. It is an object of the present invention to provide a multigrade gear oil formulation having an improved performance, particularly in gear boxes for heavy duty vehicles, and which also can be fully synthetic, although the latter is not specifically required.
It has been found that by using the complex esters as described hereinbefore as a thickener the above objects can be realised.
Accordingly, the present invention also relates to a multigrade gear oil formulation comprising:
(a) 5-45 pbw of the complex ester as described hereinbefore as a thickener,
(b) 5-45 pbw of an ester having a kinematic viscosity at 100 C of 2-10 cSt,
(c) 5-60 pbw of a mineral oil having a VI of at least 90 and/or a polyalphaolefin having a kinematic viscosity at 100 C of 4-10 cSt, and
(d) 5-15 pbw of the usual gear oil additives, the sum of the amounts of the components (a) to (d) being 100 pbw.
Components (b), (c) and (d) can be any ester, mineral oil and/or polyalphaolefin and additives known to be useful or already used in multigrade gear oil formulations.
Component (b), the low viscosity ester, may be any ester suitable for improving additive solubility and compatibility as well as for improving thermal and oxiation stability and for imparting the desired low temperature viscosity to the gear oil formulation. Preferably, component (b) is an ester of a neopentyl polyol, suitably trimethylolpropane, with at least one aliphatic, saturated monocarboxylic acid having 6 to 12 carbon atoms. An example of such ester is commercially available under the trade name PRIOLUBE 3970.
Component (c) may be a mineral oil or a PAO, which should have a VI of at least 90. It is, however, preferred to use a PAO, particularly PAO 6 and PAO 8.
Component (d) may be any available gear oil EP/AW additive package known to be useful in automotive and industrial gear oil formulations.
The complex esters may be produced in a batch or continuous process. The invention further provides a process for the manufacture of a complex ester which comprises reacting at least one polyfunctional alcohol, at least one polyfunctional carboxylic acid and a chain stopping agent, wherein
(a) the polyfunctional alcohol is a hindered or non-hindered, aliphatic polyol,
(b) the polyfunctional carboxylic acid comprises an aliphatic dicarboxylic acid containing from 9 to 18 carbon atoms, dimerised and/or trimerised fatty acids or mixtures thereof, with the proviso that dimerised and trimerised fatty acids do not constitute more than 80% by weight, preferably not more than 50% by weight, of the total amount of polyfunctional carboxylic acid used,
(c) the chain stopping agent comprises either an aliphatic monocarboxylic acid selected from the group consisting of straight chain saturated acids containing from 7 to 22, preferably from 7 to 14, carbon atoms, branched saturated acids containing from 7 to 24 carbon atoms, straight or branched unsaturated acids containing from 16 to 24 carbon atoms and mixtures thereof or at least one aliphatic, straight or branched, saturated or unsaturated, monofunctional alcohol containing at least 14 carbon atoms, and preferably not having more than 24 carbon atoms, and
(d) the complex ester has a kinematic viscosity at 100 C (Vk,100) of from 30 to 1000 cSt, preferably from 30 to 200 cSt.